Given the general desire to store ever-increasing amounts of digital information, designers and manufacturers of magnetic storage devices are continually striving to increase the bit density of magnetic storage media. In a magnetic recording disk this means increasing the areal density, i.e., both the number of tracks on a disk and the linear density of bits along a given track. New materials, as well as new recording methods, have led to higher areal densities.
For example, perpendicular magnetic recording systems have been developed for use in computer hard disk drives. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole. Perpendicular recording media typically include a hard magnetic recording layer and a soft magnetic underlayer which provide a flux path from the trailing write pole to the leading opposing pole of the writer.
To write to the magnetic recording media, the recording head is separated from the magnetic recording media by a distance known as the flying height. The magnetic recording media is moved past the recording head so that the recording head follows the tracks of the magnetic recording media, with the magnetic recording media first passing under the opposing pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole. Because the magnetic flux magnetizes the magnetic recording layer in a vertical orientation, a much higher linear density can be achieved compared with longitudinal magnetic recording systems.
Areal densities have also been increased greatly by decreasing the number of magnetic grains in one data bit and by simultaneously decreasing the size of the magnetic grains. As the grains per bit and the overall size of the bits decrease, however, media noise and lower performance characteristics can arise due to exchange coupling among the grains. In addition, the thermal stability, and thus reliability, of the magnetic recording material is lowered as the grain volume is reduced below a minimum threshold where the ratio of magnetic energy to thermal energy for a given material reaches a superparamagnetic limit.
Among the currently proposed solutions to issues such as these are heat-assisted magnetic recording (HAMR) and bit-patterned media (BPM). In BPM, the magnetic recording surface is patterned to provide a number of discrete, single-domain magnetic islands (usually one island per bit) separated from each other to decrease exchange coupling between data bits. During a writing operation, a write head must be precisely positioned over a desired bit/island to magnetize the bit. As such, the writing process must be carefully synchronized with the data bits passing by the head as the disk rotates within the drive to facilitate accurate recording and eventual readback of data. Given the small size of the bits and the high speeds of rotating disk systems, accurately positioning the head over a desired bit can be difficult.
Heat-assisted magnetic recording, or HAMR, compensates for smaller grain volumes by using magnetic recording media having a very high magnetic anisotropy. The magnetic recording medium is heated during the write process in order to lower medium's coercivity sufficiently for a write head's magnetic field to magnetize the medium. Adaptation of the write head for heating and heat dissipation in the magnetic recording medium complicate the recording process. In addition, to provide magnetic and thermal decoupling for very small grain sizes (e.g., 3-5 nanometers), the high anisotropy magnetic grains require atomically thin and sharp grain boundaries. Such features can be difficult to achieve at the high temperatures required for processing high anisotropy materials.